CHAPTER XII. 



INFECTION. 



BY infection we understand the entrance of bacteria into 

 the body and their multiplication there. Theoretically that 

 definition is correct ; but something more than the entrance 

 and multiplication of bacteria is necessary to constitute an in- 

 fection. Their presence and multiplication must be mani- 

 fested by symptoms. A person may swallow the typhoid 

 bacillus without contracting the disease. Theoretically that 

 constitutes an infection, but that is not the case practically, 

 because the germ does not manifest its presence by any dis- 

 turbance of the health of the individual. 



Some injury must be done the body by the germ before 

 there can be said to be an infection. The surface of the skin 

 is known to harbor bacteria at all times, but unless searched 

 for they remain unrecognized. They multiply there, and yet 

 there is no evidence of infection. When these same germs 

 enter a wound and produce substances which react on the body, 

 an infection is said to have occurred. It is impossible to give 

 a definition of infection that can be embraced in one sentence. 



Infection may occur not only with the vegetable organisms 

 (/'. e., bacteria, moulds, and yeasts), but also with the animal 

 parasites, such as the Amoeba coli and the malarial hematozoon. 



An infectious disease is one caused by micro-organisms, and 

 which is liable to be communicated to others. The non- 

 pathogenic bacteria are incapable of producing an infectious 

 disease. Some organisms are infectious for animals but not 

 for man, and vice versa. In the condition known as saprcemia 

 the infection is due to saprophytes which, while not entering 

 the blood themselves (i. e., remaining in the focus of infec- 

 tion), yet produce substances that are absorbed by the body 

 and enter into the blood and lymph-channels. 



By mixed infection is meant the presence of more than one 



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